6V6 line preamp

[stevenrotterdam]

Hi Salas!
Thanks for simulating, although I don't know how to interpret this graph.
Nice to see reality conforms to your calculations.
I used Russian 6v6, measurements:
Ibias 30 mA
vK 16,2 V
V plate 255 V.

Ra is 50 W vishay non-inductive audiograde blablabla so it should be OK.
I tested with input signals +/- 1,5 and 9 V (batteries), with 1,5 output was symmetrical, with 9 V something like 2% difference between pos and neg, but who needs 50 V preamp output..
Could you please do a simulation with R load DC 22k and AC 7K? (Parallel Ra 22k and Rload pot10k)
Thanks
Steven

 

[Salas]

Gives 20dB gain with 10K pot across input, same THD. The FFT I showed is a prediction of THD with 100K pot at max and input 3V peak to peak sinewave. 10K pot is rather too low IMHO. Do you need 20dB line gain? A 50K log pot would be smoother controlling I guess.

 

[Salas]

P.S. Is it a CLC supply with tube rectification? Any pictures of this 900V B+ test build?

 

[stevenrotterdam]

Supply is 230-400V transformer with voltage doubler, diodes BY 527 (old) 330 microfarad 450 Vcaps , nippon chemicon old jap crap but I hope good quality, but everything is in experimental stage so a picture would be a box with transformer caps etc.
But I dont have an input pot only an output pot.
Because of that I would like the simulation of the tube, with load 22K for DC and load & 7K for Dc, I think 7K is also a good linear load for 6v6.
If you like it I will send photo of the amp when it is nearly finished.
Steven

 

[Salas]

Does the grid have a resistive reference to ground as it is now? It can't just flap in the cool breeze?

 

[stevenrotterdam]

Sorry, meant DC load 22 K and AC load 7K

 

[stevenrotterdam]

Grid connected to ground by resistor 68 K, stopper res. 1 K

 

[Salas]

But I dont have an input pot only an output pot.
Because of that I would like the simulation of the tube, with load 22K for DC and load & 7K for Dc, I think 7K is also a good linear load for 6v6.
If you like it I will send photo of the amp when it is nearly finished.
Steven

With 10K to ground after the output capacitor the gain is 18dB but THD is about 16dB worse than having the same pot on input, so I suggest you move it to the front as in the original design.

 

[Salas]

Grid connected to ground by resistor 68 K, stopper res. 1 K

Good.

 

[stevenrotterdam]

OK thanks I'll think about it but now I need to rethink because i don't know whether the sources like to be loaded by 10K

 

[Salas]

Get a 50K log pot. Cheapish carbon ones, those with the tin body are surprisingly good tone for short term. Just load the output cap with 1Meg or 470K to ground before RCA for bleeding any small charge to avoid pop sounds when connecting up.

 

[stevenrotterdam]

OK, will test with higher-ohms-output pot. And send photos when ready
And now sleepy-time.
Steven

 

[Salas]

Just walk in a guitar spares or electronics shop in your city and there will be cheap log pots galore. Nighty nighty.

 

[stajo]

-Wich secondary value for output tx 600 ohms or other value for a SS amp? wich power in mA have to be rated? wich kind of material? suggested brands? or better order a special tx with the necessary specs?

I have a pair of http://www.lundahl.se/pdf/1660.pdf 18 mA laying around, I'm inclined to test them 4.5:1 in this application because preamps with to much gain is not fitting anywhere around my other stuff. And I'm not to fond of CFs in general. Its gonna take som fiddle around with B+ I think.

/Staffan

 

[Salas]

This pre runs near 30mA bias originally though.

 

[stajo]

This pre runs near 30mA bias originally though.

Oh, I thought it was around 22 or something for the tube. I have it on a breadboard somewhere and either I can try some different B+ and bias settings or try with another tube. I have a box of 6G6-G somewhere. A bit weaker but nice tubes running around 11 mA in triode. Rp 4750 and under 10 in amplification.

Merry Christmas
Staffan

 

[Salas]

Yes it is designed for 22mA. But it has been seen hitting higher, depends on tube brand and vintage a bit also. Merry Xmas!

 

[stajo]

Yes it is designed for 22mA. But it has been seen hitting higher, depends on tube brand and vintage a bit also. Merry Xmas!

1660 takes up to 50 easy, it just starts to saturate at a certain signal level so I think its safe to experiment. Can also try Alt T 2:4.5

Staffan

 

[DimZ]

report after some "technical" problems ...

tubes - tungsol reissue
Vin - 363V, Vout - 340
R4 set to 63mA
plate voltage 231
14.62V / 14.73V on cathode reistors

dc offset under 12mV at both channels - but still varies ,
output caps at the moment are jantzen mkp 3.6uf bypassed by teflon 0.056uf K72p6

I'll upload pics later ....

 

[Salas]

There can't be DC offset in AC capacitive coupling, only leakage, but modern capacitors have minimal. Is the DMM zeroing at DCmV when you touch its probes together? If yes, maybe those K72 are a bit dodgy? If you lift one side of those, does the DCmV change?